Dennett's
tower is set of conceptual creatures each one of which is successively
more capable of reacting to, and hence surviving in, the world through
having more sophisticated strategies for 'generating and testing'
hypotheses about how to react. This doesn't just mean deciding what
to do next when confronted with, for instance, a predator who just
might be about to eat you (options: fight, flee, play dead, attempt to
make friends, pray to your god, etc); by generate-and-test Dennett
instead refers to how the creature figures out what the options
are and then tries out each one. Read chapter 13 of Darwin's Dangerous
Idea for the full account, but here's my very brief synopsis of the
storeys of Dennett's tower, starting on the ground floor:

Darwinian creatures have only natural selection as the generate and test mechanism, so
genetic mutation and selection is the only way that Darwinian creatures
can adapt - individuals cannot.

Skinnerian creatures can learn but only by literally generating and testing all different
possible actions then reinforcing the successful behaviour (which is
ok providing you don't get eaten while testing a bad course of action).

Popperian creatures are Skinnerians that have the additional ability to internalise
(imagine) the possible actions so that some (the bad ones) are
discarded before they are tried out for real.

Gregorian creatures are tool makers including - importantly - mind tools like
language, which means that individuals no longer have to
generate-and-test all possible hypotheses since others have done so
already and can pass on that knowledge.

Scientific creatures.
Here Dennett proposes that a particular way of rigorously, collectively
and publically testing hypotheses - namely the scientific method - is
sufficiently powerful and distinct to merit a further floor of the
tower. (I'm not sure that I agree, however that isn't important to the
point I'm trying to make in this essay.)

Like
the Tower of Hanoi each successive storey is smaller (a sub-set) of
the storey below, thus all Skinnerian creatures are Darwinian, but only
a sub-set of Darwinian creatures are Skinnerian and so on. I propose here another storey to the Tower, for what I want to call Walterian Creatures,
after the pioneering neurophysiologist W. Grey Walter who, in
1948-49, invented his famous electro-mechanical robot tortoises,
acknowledged by many roboticists as the precursors of what we now
call Artificial Life. But before I explain what Walterians are I need to discuss Gregorians in more detail.

Gregorian
creatures (named after British psychologist Richard Gregory) are tool
makers, of both physical tools (like axes and wheels) and
mind-tools (like language and mathematics), and Dennett suggests
that these tools are 'intelligenceamplifiers'. Certainly they give
Gregorian creatures a significant advantage over merely Popperian
creatures, because they have the benefit of the shared experience of
others, expressed either through using the tools they have made or
refined or, more directly, through their knowledge or instructions as
spoken or written. I would argue that the most powerful intelligence
amplifier so far created by one particular species of
Gregorian-Scientific creature: man, is the computer, for with it we
are able to simulate almost any reality we can imagine. Computer
simulation is potent stuff, gedanken thought experiments are no longer doomed to remain flights of fancy and
mathematical models need no longer remain dry abstractions. And one
of the most remarkable kinds of computer simulation is of intelligence
itself: Artificial Intelligence.

What
if the tools made by Gregorian creatures take on a life of their own
and become, in a sense, independent of the tool-makers?
Embodied Artificial Intelligence (Artificial Life) has this
potential. Walterian creatures are, I propose, smart tools that have learned to think, grown up and
left the toolbox. Think of future intelligent robots (far more
capable than the crude prototypes we can currently build) that might
co-exist with humans in an extraordinary symbiosis.

The defining characteristic of Walterian creatures is that they are artificial. They've not only left the toolbox but crawled out of the gene pool.
No longer bound by the common biochemistry of Earth's biota, yet
sharing both the inheritance and evolutionary (albeit artificial)
processes of their Darwinian ancestors. So what does this mean for
Walterian creatures? Well, all of Walterian’s ancestors share the fact
that however simple or sophisticated their strategies for hypothesising
about possible actions those actions have to be undertaken by the
self-same physical creatures that do the hypothesising. Ok,
Gregorian-Scientific creatures can augment themselves with magnificent
tools for compensating for their own sensory or physical limitations,
like electron microscopes or manned spacecraft, or remotely
operated robot space probes that act as sense extenders, but one thing
Gregorian individuals cannot do is evolve themselves as part of the generate-and-test process. Consider this scenario. A
future intelligent autonomous robot is exploring a planet about which
very little is known. As part of its generate-and-test strategy this
Walterian can in simulation fast-forward artificial genetic
algorithms to evolve its own physical capabilities and then re-build
parts of itself on-the-fly to best deal with the situation it has
encountered. It could, for instance, artificially evolve and
re-engineer itself the means to make best use of whatever energy
sources are to hand. (It would be like you or I falling into a river
and being able to artificially evolve and grow gills in less time than
it takes to drown.)

Walterian
creatures are, like Gregorians, able to share tools, knowledge and
experience. They will be fully interconnected, so that any individual
- subject only to the physical delays of the networking technology -
can instantly seek information or resources from the shared Walterian
artificial culture. However, unlike Gregorians, these individuals are
capable of Lamarckian learning. Need a skill fast? If you’re a
Walterian creature then, providing at least one other individual has
already learned the skill and is either online or has previously
uploaded that skill, then you simply download it. Walterian creatures
would surely have profoundly different - and perhaps unimaginable by us
merely Gregorian creatures - kinds of minds.

This essay is a revised version of a piece published on my blog in April 2007